Discontinuous reception (DRX) mode may be provided in mobile terminals in order to reduce power usage in both idle and connected mode, such as e.g. by allowing the mobile terminal to periodically disable the receiver and entering a power saving mode. However, in certain scenarios the network may require a mobile terminal to extend the “onTime” of the UE in Connected-DRX (C-DRX) mode, i.e. the time period in which the receiver of the UE must be active in order to receive transmission from a base station. In particular, the receiver must remain active during an uplink (UL) Hybrid Automatic Repeat Request (HARQ) process, during which a UE transmits an message to a base station and attempts to receive any subsequent retransmission requests. As specified by the 3rd Generation Partnership Project (3GPP) standard, the onTime may extended in an uplink HARQ process when the HARQ transmission buffer (i.e. the buffer containing uplink data to be transmitted) is not empty and there is a possibility to receive a retransmission grant from a serving base station.
The serving base station may send such a retransmission grant in the event that the reception of an uplink data message from a mobile terminal was unsuccessful. The serving base station may request retransmission of the uplink data message from the mobile terminal by transmitting the retransmission grant, to which the mobile terminal may respond by retransmitting the uplink data message (i.e. as stored in the HARQ transmission buffer).
The serving base station may transmit retransmission requests according to a periodic schedule following the initial uplink grant (i.e. the grant from the serving base station initially instructing the mobile terminal to send the data message), such as e.g. every 8 ms following the initial uplink grant. Accordingly, the mobile terminal may be configured to monitor for retransmission grants at times t=8, 16, 24, . . . ms following reception of an initial uplink grant at time t=0. Each of the time periods corresponding to t=8, 16, 24, . . . ms when a retransmission grant may be received by the mobile terminal may thus be referred to as a retransmission grant occasion.
The mobile terminal may thus be required to listen during each retransmission grant occasion for potential retransmission grants, regardless if an acknowledgement (ACK) message indicating successful receipt of the uplink data message is received. In other words, even if the serving base station indicated through HARQ feedback (i.e. by transmitting an ACK) that the uplink data message was successfully received, the mobile terminal may still be required to listen for retransmission grants during one or more subsequent retransmission grant occasions.
The network may define a maximum number of total HARQ transmissions including the initial uplink grant and any remaining retransmissions with the parameter maxHARQ_tx. As maxHARQ_tx defines the total number of HARQ transmissions including the initial uplink grant, a total of maxHARQ_tx−1 retransmission grant occasions may occur following the initial uplink grant. The mobile terminal may thus be required to listen every 8 ms for potential retransmission requests during each of the maxHARQ_tx−1 retransmission grant occasions. As each of these grants requires reception of a message by the mobile terminal, the receiver will need to be active. Consequently, the mobile terminal may not be able to enter a low power state during the retransmission grant occasions.
The 3GPP standard has defined the default value of maxHARQ_tx as 5, and accordingly a mobile terminal is required to listen during the 4 retransmission grant occasions (i.e. maxHARQ_tx−1 retransmission grant occasions) following reception of an initial uplink grant. Accordingly, a mobile terminal operating according to maxHARQ_tx=5 must listen for potential retransmission grants at t=[8, 16, 24, 32] ms. Furthermore, the standard dictates that the mobile terminal must monitor for retransmission grants during every retransmission grant, regardless if an ACK message is received. For example, a mobile terminal may receive an ACK message at t=8 ms following an initial uplink transmission, indicating that the serving base station has successfully received the initial uplink transmission and does not require any retransmissions. However, the mobile terminal is still required by the 3GPP standard to listen at t=[16, 24, 32] ms. Accordingly, onTime for the mobile terminal is significantly increased during an uplink HARQ process. Use of power saving options such as power/clock gating of temporarily not required modem subcomponents may not be possible.
In addition to a default value of maxHARQ_tx=5, maxHARQ_tx may be set by the network by Radio Resource Control (RRC) signaling, and may be e.g. set to any value out of the set {1, 2, 3, 4, 5, 6, 7, 8, 10, 12, 16, 20, 24, 28}. While any of these values are possible, it is common practice to use the default value of 5. Additionally, the default value of maxHARQ_tx=5 will be assumed if no value for maxHARQ_tx is directly specified by the network.